Earthmoving and construction work machines often employ hydraulic systems that provide functionality and control to various aspects of the machines. For example, some work machines employ hydraulic braking systems to control driving speeds and fan hydraulic drive systems that-control machine cooling.
Because each system may have separate flow requirements, the hydraulic systems on some work machines are isolated systems, each having a separate fluid pump. However, providing a separate pump for each separate hydraulic system may be redundant and may be cost prohibitive. Further, such a system may require additional maintenance and upkeep. To overcome these disadvantages, some work machines combine or integrate certain hydraulic systems.
Even in a combined system, one hydraulic system may require different fluid flow parameters than the other and, in addition, may require the different fluid flow at different times. To address this, some known systems direct fluid from a common pump to one system or the other system using a cut-in/cut-out device. For example, on a conventional work machine having a combined braking system and a combined cooling system, fluid may be directed to the brake system to charge or fill accumulators with fluid. Then, the cut-in/cut-out device may cut-out fluid flow to the braking system and may instead direct fluid to the cooling system. Through normal braking, the fluid in the accumulators may be gradually expended until the volume or pressure falls below a lower threshold. When this occurs, the cut-in/cut-out device may cut-in fluid flow to the braking system to again charge the accumulators, while cutting-out fluid flow to the cooling system. Because of this, the fluid level in the accumulators of the brake system fluctuates between a relatively high level and a relatively lower level. However, even at the lower level, the accumulators on conventional work machines must provide a sufficient volume of fluid to stop the work machine in the event of a pump failure.
Another known system that combines a brake and a cooling system is disclosed in U.S. Pat. No. 6,681,568 to Smith. The '568 patent discloses a system having a relief valve in a fluid line directed to a cooling circuit. Control of the relief valve affects fluid flow to both a braking circuit and the cooling circuit. While providing effective cooling and braking, the system disclosed in the '568 patent may be improved upon. For example, the accumulators in the '568 patent may be sized to allow fluctuation between a high fluid level and a relatively lower fluid level, with the accumulators being sized to provide sufficient fluid to the brakes even when the fluid level in the accumulators is at the lower level. Such accumulators may be larger than necessary. Therefore, the system in the '568 patent may be improved upon to provide smaller accumulators that may result in space and cost savings.
The electro-hydraulic system disclosed and described herein may overcome one or more of the problems in the prior art.